Sn=(a1+an)*n/2=(-19+2n-21)*n/2=n²-20n
an=2n-21
∴ n≥11時(shí),an>0
n≤10時(shí),an
已知數(shù)列{an}的通項(xiàng)公式an=2n-21,前n項(xiàng)和sn．如果bn=|an|（n∈N）,求數(shù)列{bn}的前n項(xiàng)和Tn．
已知數(shù)列{an}的通項(xiàng)公式an=2n-21,前n項(xiàng)和sn．如果bn=|an|（n∈N）,求數(shù)列{bn}的前n項(xiàng)和Tn．
數(shù)學(xué)人氣：415 ℃時(shí)間：2020-06-23 20:27:32
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